The switched reluctance (SR) motor has become widely used for its high efficiency and flexible control characteristics. The SR motor does not require permanent magnets and produces torque by the variable reluctance principle. In operation of the SR motor, it is necessary to synchronize phase excitation pulses to the position of the rotor. The rotor position information is thus necessary to determine the switching instants for proper control of speed, torque, and torque pulsations.
Previously, shaft position transducers have been commonly employed to determine the rotor position in SR motors. Such transducers have included a magnetized ring along with Hall effect sensors or of opto-interrupters with a slotted disk that produces pulses at particular intervals. Such position sensors, however, are relatively expensive and may tend to reduce the system reliability due to electromagnetic interference and temperature effects.
An indirect method of rotor position sensing is described in Macminn et al., "Application of the Sensor Integration Techniques to the Switched Reluctance Motor Drive," IEEE Industry Applications Conference Record 1988, pp. 584-588. In this technique, short duration, low level voltage pulses derived from the commutating signals are applied to the two unenergized phases of an SR motor and the resulting current pulses are measured to obtain an indication of the impedances of the unenergized motor phases and an estimation of the rotor angle. The circuitry for implementing this method, however, is rather complex and relatively expensive.
Other techniques for indirectly sensing the position of an SR motor include dwell angle modulation in an "open loop" system, as described in J. T. Bass, et al., "Robust Torque Control of Switched Reluctance Motors Without a Shaft Position Sensor", IEEE Trans. on Ind. Elec., Vol. 1E-33, No. 3, pp. 212-216, August 1986; and in J. T. Bass, et al., "Simplified Electronics for Torque Control of Sensorless Switched Reluctance Motor", IEEE Trans. on Ind. Elec., Vol. 1E-34, No. 2, pp. 234-239, May 1987. Further, U.S. Pat. No. 5,072,166 by the present applicant, entitled "Position Sensor Elimination Technique for the Switched Reluctance Motor Drive", issued Dec. 10, 1991, describes an indirect rotor detection technique which senses the motor phase inductance by generating a frequency modulated signal from an oscillator connected to the stator windings.
However, each of the above-noted prior art techniques have not been wholly satisfactory due either to excessive complexity, inaccuracies caused by induced parasitic currents, or lack of fully adequate running performance over a wide range of torque load and inertia. A need has thus arisen for a simple, cost-effective, technique to detect the rotor position of an SR motor.